Backup's begin version is lower than the actual version

While working on the new backup system, I found that current backup's `properties/log_begin_version` specifies a version that is smaller than the actual beginning version. The cause is that this version is chosen first in a transaction, and then a second transaction starts the backup. The actually mutations are logged after the second transaction is executed. As a result, mutations between these two transactions are missing.

This doesn't affect correctness, since range files and mutation logs together can still provide a consistent snapshot of the database, as they are recorded after the second transaction.

Specifically, the first transaction to get read version is here: https://github.com/apple/foundationdb/blob/1700d479c9f0047a0e6ecfa665b9ba34f3f18d2f/fdbclient/FileBackupAgent.actor.cpp#L2390

The second transaction is: https://github.com/apple/foundationdb/blob/1700d479c9f0047a0e6ecfa665b9ba34f3f18d2f/fdbclient/FileBackupAgent.actor.cpp#L2415

Read fdbclient/FileBackupAgent.actor.cpp at the two linked transaction sites around lines 2390 and 2415, then trace how their read versions populate properties/log_begin_version. Confirm the intended relationship between the first read version, the backup-start transaction, and subsequent mutation logging. Done means the reported begin version reflects the actual backup boundary without omitting mutations; no test path is mentioned.

Written by the indexing model from the issue text.